linux-kernel - Re: [PATCH v2 1/2] leds: rgb: leds-qcom-lpg: Fix pwm resolution max for Hi-Res PWMs

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Message-ID: <c328c258-6e26-4727-98d8-c9d551358bbd@quicinc.com>
Date: Wed, 26 Feb 2025 17:47:55 -0800
From: Anjelique Melendez <quic_amelende@...cinc.com>
To: Abel Vesa <abel.vesa@...aro.org>, Lee Jones <lee@...nel.org>,
        Pavel Machek
	<pavel@...nel.org>
CC: Kamal Wadhwa <quic_kamalw@...cinc.com>,
        Jishnu Prakash
	<jishnu.prakash@....qualcomm.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Johan Hovold <johan@...nel.org>, Sebastian Reichel <sre@...nel.org>,
        Pavel Machek <pavel@....cz>, <linux-leds@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] leds: rgb: leds-qcom-lpg: Fix pwm resolution max
 for Hi-Res PWMs



On 2/26/2025 5:58 AM, Abel Vesa wrote:
> Ideally, the requested duty cycle should never translate to a PWM
> value higher than the selected resolution (PWM size), but currently the
> best matched period is never reported back to the PWM consumer, so the
> consumer will still be using the requested period which is higher than
> the best matched one. This will result in PWM consumer requesting
> duty cycle values higher than the allowed PWM value.
> 
> Currently, the consumer driver known to fail this way is the PWM backlight
> (pwm_bl) and should be reworked in such a way that the best matched period
> is used instead.
> 
> As for the current implementation of the duty cycle calculation, it is
> capping the max value, fix that by using the resolution to figure out the
> maximum allowed PWM value.
> 
> Cc: stable@...r.kernel.org    # 6.4
> Fixes: b00d2ed37617 ("leds: rgb: leds-qcom-lpg: Add support for high resolution PWM")
> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> ---
>   drivers/leds/rgb/leds-qcom-lpg.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/leds/rgb/leds-qcom-lpg.c b/drivers/leds/rgb/leds-qcom-lpg.c
> index f3c9ef2bfa572f9ee86c8b8aa37deb8231965490..146cd9b447787bf170310321e939022dfb176e9f 100644
> --- a/drivers/leds/rgb/leds-qcom-lpg.c
> +++ b/drivers/leds/rgb/leds-qcom-lpg.c
> @@ -529,7 +529,7 @@ static void lpg_calc_duty(struct lpg_channel *chan, uint64_t duty)
>   	unsigned int clk_rate;
>   
>   	if (chan->subtype == LPG_SUBTYPE_HI_RES_PWM) {
> -		max = LPG_RESOLUTION_15BIT - 1;
> +		max = BIT(lpg_pwm_resolution_hi_res[chan->pwm_resolution_sel]) - 1;
>   		clk_rate = lpg_clk_rates_hi_res[chan->clk_sel];
>   	} else {

I have a patch under review, 
https://lore.kernel.org/all/20250213003533.1684131-1-anjelique.melendez@oss.qualcomm.com/, 
which adds support for 6-bit resolution for regular PWM so this capping 
problem will also become an issue for regular PWM.
I think it would make sense for you to include fixing the max for 
regular PWM here. Thoughts?

>   		max = LPG_RESOLUTION_9BIT - 1;
>